This invention relates to electric power steering devices for automobiles, and more particularly to a control circuit for electric power steering devices having a clutch for connecting and disconnecting the assisting electric motor to and from the steering shaft.
Automotive, vehicles generally comprise a steering wheel by means of which the operator of the vehicle maneuvers the course of the vehicle and turns it to the right or to the left; the torque applied by the operator on the steering wheel is transmitted, via a steering shaft, etc., to the steered wheels of the vehicle to turn the course of the vehicle to the right or to the left. To assist the operator of the vehicle in turning the steering wheel, most automotive vehicles today use power assist mechanisms. Among them, hydraulic power assist mechanisms have been most common. However, electric power steering devices, which utilize an electric motor for providing an auxiliary power to assist the operator of the vehicle, are now gaining an increasing acceptance; they promise to be more economical, light-weight, and reliable. Further, it is easy to use sophisticated electronic control circuitry in the case of electric power steering devices.
FIG. 1 shows a typical organization of the control circuitry for controlling the direction and magnitude of the torque of the electric motor of an electric power steering device. The circuitry comprises: a torque sensor 1 for detecting the torque applied by the operator of the vehicle; an electric motor 14, coupled to the steering shaft through a reduction gear, etc., for providing an auxiliary torque to assist the operator in turning the steering shaft; and a control circuit 16 which controls, in response to the steering torque detected by the torque sensor 1, the direction and the amount of the motor current supplied to the motor 14 so that a proper assisting torque may be provided by the motor 14.
The torque sensor 1 is coupled to the steering shaft (not shown) of the vehicle to sense the torque applied thereto by the operator of the vehicle via the steering wheel. FIG. 3 shows the relationship between the applied torque and the output of the torque sensor 1, wherein the steering torque T applied by the operator is plotted along the abscissa, while the output voltage V of the torque sensor 1 is plotted along the ordinate; the point T.sub.0 on the abscissa corresponds to the neutral torque point at which the torque applied by the operator is null; the points on the abscissa to the right of point T.sub.0 represent steering torques to turn the vehicle to the right; on the other hand, those points which are situated to the left of point T.sub.0 represent steering torques to the left. The relationship between the steering torque T and the output voltage V is substantially linear as shown in FIG. 3: to the neutral or null torque point T.sub.0 corresponds an output V.sub.0 ; to left steering torques T.sub.1 and T.sub.4 correspond outputs V.sub.1 and V.sub.4, respectively; and to right steering torques T.sub.2 and T.sub.3 correspond outputs V.sub.2 and V.sub.3, respectively.
In response to the output V of the torque sensor 1, the control circuit 16 controls the output P of the electric motor 14 as illustrated in FIG. 4, wherein the output voltage V of the torque sensor 1 is plotted along the abscissa while the output P of the motor 14 is plotted along the ordinate.
When the output of the torque sensor 1 is in the region between V.sub.1 and V.sub.2, the output P of the motor remains null; namely, when the steering torque T applied by the operator is in the region between points T.sub.1 and T.sub.2 (see FIG. 3), no current is supplied to the motor 14 to energize it. Thus, the region of the steering torque T between the points T.sub.1 and T.sub.2, or the region of the sensor output V between the points V.sub.1 and V.sub.2 corresponding thereto, constitutes the non-sensitive region in which no assisting power is provided by the motor 14.
When the steering torque T in the right steering direction becomes greater than the level represented by point T.sub.2, an auxiliary torque in the same steering direction is provided by the motor 14 in the following manner: after the steering torque T exceeds point T.sub.2 to raise the output voltage V of the torque sensor 1 above the level V.sub.2 (as shown in FIG. 3), the output P of the motor 14 is increased substantially linearly with respect to the sensor output V as shown in FIG. 4, until the steering torque T reaches point T.sub.3 to raise the sensor output V to a saturation level V.sub.3 ; after the right steering torque T exceeds point T.sub.3, the motor output P is held at the constant level Pmax. Thus, between the points T.sub.2 and T.sub.3, an assisting torque to the right which is substantially proportional to the steering torque T is provided by the motor 14; above point T.sub.3, the assisting torque provided by the motor 14 is saturated, i.e. is held at a predetermined maximum level Pmax.
When, on the other hand, the steering torque T in the left steering direction becomes greater than the level represented by point T.sub.1 (i.e. when steering torque T is in the region to the left of the point T.sub.1 in FIG. 3), an auxiliary torque in the same (i.e. left) steering direction is provided by the motor 14 in a manner symmetrical to the above case of right steering direction. Namely, when the magnitude of the left steering torque T increases from level T.sub.1 to level T.sub.4 (i.e. moves to the left from point T.sub.1 to point T.sub.4 on the abscissa in FIG. 3), the sensor output V decreases from V.sub.1 to V.sub.4 ; in the region between V.sub.1 and V.sub.4, the output P of the motor 14 is substantially linear to the sensor output V as shown in FIG. 4, so that an assisting torque substantially proportional to the steering torque T is provided by the motor 14 in this region; when the steering torque T is above the left saturation point T.sub.4 corresponding to the sensor output V.sub.4, the motor output P is held at the constant maximum level Pmax. Thus, the left linear control region between sensor outputs V.sub.1 and V.sub.4 corresponds to the right linear control region between sensor outputs V.sub.2 and V.sub.3 ; the left saturated control region below sensor output V.sub.4 corresponds to the right saturated control region above sensor output V.sub.3.
The output of the motor 14 is controlled in response to the output of the torque sensor 1 in the above manner by the control circuit 16. Referring again to FIG. 1, let us now describe the organization and method of operation of the control circuit 16, which comprises a torque signal interface circuit 2 coupled to the output of the torque sensor 1 through input terminals 15A and 15B of the control circuit 16, and a microcomputer 3 supplied with the torque signal (i.e. the output of the torque sensor 1) via the interface circuit 2. The microcomputer 3 determines the direction and the magnitude of the assisting torque which are to be provided by the motor 14, on the basis of the torque signal outputted from the sensor 1; namely, in response to the input signal from the torque signal interface circuit 2, which signal corresponds to the above output voltage V of the torque sensor 1, the microcomputer 3 determines the rotational direciton of the motor 14 corresponding to the steering torque T, and the output P of the motor 14 corresponding to the sensor output voltage V, on the basis of the relationships shown in FIGS. 3 and 4, respectively. The direction and the magnitude of the output torque of the motor 14 are controlled by the motor driver circuit 10 in accordance with these determinations of the microcomputer 3, as described in the following.
The right and the left direction signal from the microcomputer 3 are outputted to the motor driver circuit 10 through the right and left interface circuits 4 and 5, respectively. Namely, when the steering torque T is in the right steering direction and hence the sensor output voltage V is above the level V.sub.0 corresponding to the neutral steering torque T.sub.0, a right direction signal is outputted from the microcomputer 3 to the motor driver circuit 10 through the right direction signal interface 4; when, on the other hand, the steering torque T is in the left steering direction and hence the sensor output voltage V is below the level V.sub.0, a left direction signal is outputted from the microcomputer 3 to the motor driver circuit 10 through the left direction signal interface 5.
On the other hand, the digital signal indicating the motor output torque P, which corresponds to the sensor output level V with respect to the relationship shown in FIG. 4, is outputted from the microcomputer 3 to the digital-to-analog converter 6, wherein it is converted into an analog signal indicating the motor output P; in response to this analog signal outputted from the digital-to-analog converter 6, a pulse width modulation circuitry (including an error amplifier 7, pulse width modulator 8, reference frequency oscillation circuit 9, a motor current detector circuit 11, and a motor current detecting resistor 12) generates pulses at a predetermined frequency whose pulse width varies in proportion to the magnitude of the motor output level P determined by the microcomputer 3. This pulse width modulation is effected in the following manner: Namely, the resistor 12 develops thereacross a voltage corresponding to the amount of the motor current supplied to the motor 14 from the motor driver circuit 10; the voltage developed at the output terminal of the detecting resistor 12 corresponding to the amount of the motor current is supplied to the motor current detector circuit 11, which in its turn outputs a signal corresponding to the amount of the motor current; the detecting circuit 11, however, limits its output under a predetermined level, or cuts off its output, when the voltage outputted from the resistor 12 exceeds a predetermined level. The error amplifier 7 compares the output of the digital-to-analog converter 6 and that of the motor current detector circuit 11, and amplifies the difference therebetween; namely, the amplifier 7 outputs a signal corresponding to the error of the motor current with respect to the motor output level P determined by the microcomputer 3. In response to the output of the amplifier 7, the pulse width modulator 8 modulates the width of the pulses outputted therefrom, on the basis of the output of the oscillation circuit 9 which oscillates at a predetermined frequency. The pulse width modulator 8 outputs a pulse train whose width varies substantially in proportion to the motor output level determined by the microcomputer 3.
In response to the pulse width modulated signal from the modulator 8 and a direction signal from the direction signal interface circuits 4 and 5, the motor driver circuit 10 supplies a current corresponding to these signals to the motor 14 through the output terminals 13A and 13B of the driver circuit 10: the direction of the current supplied from the driving circuit 10 to the motor 14 corresponds to the right or the left direction signal received from the interface circuit 4 or 5; the on-time thereof, on the other hand, corresponds to the duty factor of the pulse train outputted from the modulator 8. Namely, when a right direction signal is received from the interface circuit 4, the driver circuit 10 supplies the motor current in a direction wherein the motor 14 generates a torque to turn the steering shaft to the right, thereby assisting the turning operation of the operator of the vehicle to the right direction; when, on the other hand, a left direction signal is received from the interface 5, the driver circuit 10 supplies the motor current in the other direction wherein the motor 14 generates a torque to turn the steering shaft to the left. Further, the driver circuit 10 generally comprises switching power transistors which are turned on and off in response to the pulses outputted from the modulator 8, so that the on-time of the motor current corresponds to the pulse width of the pulses outputted from the modulator 8. Thus, the output of the motor 14 is controlled to the level determined by the microcomputer 3 on the basis of the output signal of the torque sensor 1.
No clutch mechanism is shown in FIG. 1; however, an electrical power steering device for automotive vehicles may comprise a clutch which connects and disconnects the output shaft of the electric motor 14 to and from the steering shaft. Thus, FIG. 2 shows an electromagnetic clutch 25 and a clutch driver circuit 23, in addition to the elements shown in FIG. 1. In the case of the power steering device shown in FIG. 2, an electromagnetic coil of the clutch 25 is energized by the current supplied from the clutch driver circuit 23 through output terminals 24A and 24B of the driver circuit 23, so that the clutch 25 connects the output shaft of the motor 14 to the steering shaft. The clutch driver circuit 23 supplies the energization current to the clutch 25 in response to the energization signal which is outputted from the microcomputer 3 when the key or ignition switch of the vehicle is turned on. Otherwise, the organization and method of operation of the circuit of FIG. 2 are identical to those of the circuit of FIG. 1, except that the torque signal interface circuit 2 shown in FIG. 1 is omitted from the representation of FIG. 2.
The power steering devices as described above, however, suffer from the following disadvantage. Namely, when the microcomputer 3 outputs a wrong direction signal, due to exterior noises, etc., the motor 14 is rotated in the wrong direction; if this happens, the motor acts against the intention of the operator of the vehicle by supplying a torque in a direction opposite to that of the torque applied by the operator; in the worst case, the operator may lose control over the course of the vehicle. Equally dangerous is the situation in which the motor 14 is oscillated due to the malfunctioning of the microcomputer 3.